| Qiongdongnan Basin, a passive continental deep-water basin in the north of the South China Sea, is an
important natural gas base in China. The deep Oligocene Lingshui Formation in Qiongdongnan Basin represent a primary
target for forthcoming exploration in the South China Sea, however, due to limited exploration efforts, the sedimentary
system of the Lingshui Formation remains inadequately characterized. Based on core, logging and 3D seismic data, the
sedimentary characteristics of Lingshui Formation in northern slope of Baodao Sag, Qiongdongnan Basin are
systematically dissected. It is clear that Lingshui Formation can be subdivided into two third-order sequences. The
sedimentation of the 3rd member of Lingshui Formation is a complete three-order sequence with low, transgressive and
high system domains. Under the control of the fault co directional transition zone and transition slope formed by the
differential activity of F12 fault, several large differentiated river delta groups of different scales in the west and east are
developed in the 3rd member of Lingshui Formation. The fault step zone mainly developed the underwater distributary
channel of braided river delta front in the late low and high system tracts, and it is a high-quality reservoir development
interval with the thickness of sand body being large, the particle size being coarser, the separation being better, and the
physical property being better. Therefore, the development model of multistage fault-controlled material source and
transition fault-controlled sand body distribution of the 3rd member of Lingshui Formation in the northern fault step zone
of Baodao Sag is established. The high quality reservoir of large-scale delta distributary channel of the 3rd member of
Lingshui Formation directly covers the source rock of Yacheng Formation and forms a favorable transport system with
source rock-linked fault. And it is covered by thick transgressive shallow-sea mudstone deposited in late depression
period. High-quality reservoer-cap combination and transport system are the key to forming large gas fields in Baodao
Sag, and are worth of more attention in the future exploration. |
